In the fields of optical communication or optical measurement, optical waveguide elements having optical waveguides on a surface of an dielectric substrate are widely used. Among the optical waveguide elements, an optical modulator having a Mach-Zehnder type waveguide to perform optical modulation such as light intensity modulation is widely used due to advantages such as readiness of integration and high efficient optical modulation.
In the optical modulator having the Mach-Zehnder type waveguide (hereinafter, referred to as a “MZ-type waveguide”), an electric field is applied to at least an arm (branching waveguide) in the MZ-type waveguide to control phases of the light waves propagating through the corresponding arm. In addition, the LiNbO3 substrate is apt to be subject to a drift phenomenon, in which an operational point of the modulation signal is shifted by change of temperature or DC bias control over a long time period. For this reason, as disclosed in PTLs 1 to 3, the DC bias applied to the optical modulator is adjusted to find a suitable operational point by monitoring output light from the optical modulator or radiation-mode light radiated from a Y-multiplexer of the MZ-type waveguide.
As shown in FIG. 1(a), in an ideal Y-multiplexer of the MZ-type waveguide, a gap between the branching waveguides in a crotch portion of the Y-multiplexer where two branching waveguides 1 are combined becomes zero so that radiation-mode light (high-order light) is radiated in a place where the shape is changed from the coupling portion 2 to the output waveguide 3 (the boundary between the regions B and C). In addition, it is possible to determine the modulation condition of the optical modulator by monitoring the radiation-mode light (refer to PTL 2).
However, in a shape of the optical waveguide in practice, the gap G between the branching waveguides is rarely set to zero as shown in FIG. 1(b). This is because the minimum line width to form the optical waveguide is finite. Due to the influence of such a gap, in a place where the branching waveguides 1 are combined (the boundary between the regions A and B), a mode mismatching between the light waves is generated to produce so-called mode mismatching light, by which a part of the light waves propagating through the waveguide leaks.
The mode mismatching light causes degradation of optical characteristics of the optical modulator. Particularly, because of increasing propagation loss and degradation of an extinction ratio, or since the mode mismatching light interferes with the radiation-mode light, or the mode mismatching light itself is detected by monitoring means, it is disadvantageously difficult to accurately detect the radiation-mode light.
Meanwhile, some attempts have been made to thin the dielectric substrate used in the optical modulator to be equal to or smaller than 20 μm to reduce a drive voltage for driving the optical modulator or matching velocities between the propagation light and the drive signal. However, as disclosed in PTL 4, since the light waves leaking from the optical waveguide propagate through the thinned dielectric substrate while they are constricted within the substrate, it is difficult to separate the radiation-mode light from the mode mismatching light and, in some cases, it is also difficult to separate the signal light from the radiation-mode light.
Furthermore, in the thinned optical modulator, a typical line width of 5 to 7 μm of the optical waveguide is reduced to approximately 2 to 4 μm (the thickness of the substrate reaches several hundreds of micrometers). Therefore, the influence of the gap Gin the coupling portion (Y-multiplexer) of the branching waveguide increases in comparison with that of typical portions so that generation of the mode mismatching light becomes significant.
In PTL 4, the applicant discloses a method of forming the Y-multiplexer of the MZ-type waveguide in a 2×3 branching waveguide in order to separate the radiation-mode light from the signal light. However, in the configuration of the 2×3 branching waveguide disclosed in PTL 4, problematically, a part of the mode mismatching light generated when two branching waveguides are combined may be recombined with the optical waveguide and mixed with the radiation-mode light or the output light.